The present invention relates to a belt type fixing apparatus used in, for example, a copying machine, a printer, and a facsimile, particularly, to a belt type fixing apparatus for fixing an unfixed toner layer on a recording medium.
As known well, an electrophotographic image formation in, for example, a copying machine is performed by thermally fusing a powdery toner on a recording medium such as a paper sheet or a plastic sheet with a heat roll so as to fix the toner image on the recording medium.
In general, a recording medium bearing a toner image is passed through a clearance between a fixing roll comprising a mandrel and a halogen lamp arranged within the mandrel and a pressing roll so as to fuse and fix the toner under heat and pressure. Since the roll temperature is set at 200 to 250.degree. C. and it is necessary to ensure a nip width of several millimeters, a heat resistant silicone rubber layer is formed on the circumferential surface of the mandrel of the fixing roll, and the silicone rubber layer is coated with a fluoroplastic resin layer so as to ensure release properties from the toner image.
In the conventional fixing roll of the particular construction, however, the mandrel is covered with a silicone rubber layer having a hardness of about 30.degree. (Shore A) in a thickness of about 3 mm, leading to a poor heat conductivity and, thus, to a low fixing rate. Particularly, when it comes to a color copying machine or a color printer, it is necessary to develop color by completely fusing the toner in terms of the color reproduction, leading to requirement of the contact time and heating time about 3 times as long. as those for the fixing of a black toner image. Naturally, it is strongly required to shorten the fixing time.
Some proposals have been made in an attempt to shorten the fixing time, as follows: